Fix issue in Firefox related to indexDB

I got an error in firefox similar to below code

```
TypeError: setting a property that has only a getter
var indexedDB = window.indexedDB||window.mozIndexedDB||window.webkitIndexedDB||w...
```
See to confirm this code the commit https://github.com/axemclion/IndexedDBShim/commit/3b66b42cde1585514b446cbb13c0cca03e42eb77 and also this discussion https://stackoverflow.com/questions/24903851/angular-indexeddb-issues-with-firefox#answer-24922214